1. Field of the Invention
The present invention relates to a method for producing a tubular body made of fiber reinforced resin and the tubular body, made of the fiber reinforced resin, which is produced by the method. More particularly, the present invention is intended to prevent a hoop prepreg of the tubular body from being defectively layered on a base prepreg.
2. Description of the Related Art
In recent years, as a material of the tubular body such as a golf club shaft, a fishing rod, and the like, fiber reinforced resin such as carbon prepreg which is lightweight and has a high specific strength and a high rigidity is mainly used.
The tubular body made of the fiber reinforced resin is formed by layering fiber reinforced prepregs one upon another. The fiber reinforced prepregs have different characteristics according to the orientation direction of reinforcing fibers thereof as described below.
In a bias prepreg whose reinforcing fibers are inclined to the axial direction of the tubular body, the reinforcing fibers extend spirally. Therefore the bias prepreg is capable of enhancing mainly the torsional rigidity and torsional strength of the tubular body.
In a straight prepreg whose reinforcing fibers are disposed almost parallel with the axial direction of the tubular body, the reinforcing fibers extend in the axial direction of the tubular body. Therefore the straight prepreg is capable of enhancing mainly the bending rigidity and bending strength of the tubular body.
In a hoop prepreg whose reinforcing fibers are disposed almost perpendicularly to the axial direction of the tubular body, the reinforcing fibers extend in the circumferential direction of the tubular body. Therefore the straight prepreg is capable of enhancing mainly the crushing rigidity and crushing strength of the tubular body.
In producing the tubular body, in a known technique, a rigidity and a strength necessary for the tubular body are adjusted by using the bias prepreg, the straight prepreg, and the hoop prepreg in combination without increasing the weight of the tubular body.
For example, as shown in FIG. 5, the golf club shaft 1 disclosed in Japanese Patent Application Laid-Open No. 8-131588 (patent document 1) has the bias layer 2 consisting of the bias prepreg disposed at the outer side, the straight layer 3 consisting of the straight prepreg disposed inward from the bias layer 2, and the hoop layer 4 consisting of the hoop prepreg disposed inward from the straight layer 3. Description is made in the specification that the construction of the golf club shaft 1 of the patent document 1 is capable of enhancing the torsional rigidity and bending rigidity thereof without increasing the weight thereof.
But when the hoop prepreg is wound round the core, the hoop prepreg is restrained from being curved by the elasticity of the reinforcing fiber extended in the circumferential direction of the golf club shaft 1. Thus it is difficult to wind the hoop prepreg round the core. Therefore it is difficult to adherently layer the hoop layer on the underlayer, and an end of the hoop prepreg is liable to peel off the underlayer.
When the peel-off occurs, the hoop layer lifts and the hoop prepreg bends in a work of overlaying the outer layer on the hoop layer, and in addition, air is collected in a portion where the hoop layer has lifted. Thereby the hoop layer has a low strength and there occurs a variation in the characteristic of the tubular body, which leads to deterioration of the quality thereof. Further it is necessary to perform a work of adhering the defective portion of the hoop prepreg to the underlayer again, which leads to deterioration of the productivity thereof.
To solve the above-described problem, in a known technique, the straight layer is overlaid on the hoop layer and the number of turns (number of plies) of the straight layers is increased to prevent defective layering of the hoop layer by the wrapping effect of the straight layer. But in recent years, the tubular body such as the golf club shaft is strongly demanded to have a lightweight. Thus it is necessary not to form the straight layer excessively. Further to prevent the tubular body from deteriorating its strength by making it lightweight, it is conceivable that the number of the hoop layers inevitably increases. Thus the number of turns of the straight layers is demanded to be equal to that of the hoop layers at most.    Patent document 1: Japanese Patent Application Laid-Open No. 8-131588